In semiconductor processes such as etching and chemical vapor deposition, chemically reactive gases can be used during processing or produced as a result of processing. These toxic or otherwise harmful process gases can be treated or abated before their release to atmosphere. A number of technologies have traditionally been used for the abatement of such gases. For example, thermal abatement processes are commonly implemented by burning halogen-containing gases with a hydrogen-containing fuel to convert fluorine (F) or chlorine (Cl) to hydrogen fluoride (HF) or hydrogen chloride (HCl). In those exemplary processes, the resulting HF or HCl can be subsequently removed in a wet scrubber.
Another example of current abatement processes is catalytic thermal abatement. Catalytic thermal abatement is typically performed by exposing halogen-containing gases to metal oxides at high temperatures to convert halogen to salts. Another example is plasma abatement, which can be performed at sub-atmospheric pressures, or at atmospheric pressure using microwave plasmas.
In current thermal abatement processes, energy usage can be inefficient. Many of the halogen-containing compounds can be chemically stable, requiring temperatures (e.g., of 1000K to 2000K) to achieve thermal abatement. For example, this difficulty can occur in the abatement of highly stable carbon tetrafluoride (CF4).
Current thermal abatement processes conducted at atmospheric pressure can require large amounts of purge gas to be added at vacuum pumps of the plasma device for protection of the pumps. As a result, a high level of energy can be wasted just in heating the purge gas.
Catalytic thermal abatement processes can achieve higher abatement efficiency, but still can suffer from high maintenance cost and high consumable cost. During semi-conductor processing the flow of halogen-containing gases is often not continuous, toggling on and off in each wafer cycle. Since the thermal time constant of a burner to reach operating temperature can be much longer than a wafer cycle, a thermal abatement unit is often kept on continuously, dramatically reducing the energy efficiency.
In plasma abatement, existing technologies have demonstrated high abatement efficiency (e.g., greater than 95%). Plasmas can be generated in various ways including DC discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by electrostatically or inductively coupling energy from a power supply into a plasma. Parallel plates are typically used for electrostatically coupling energy into a plasma. Induction coils are typically used for inducing current into a plasma. Microwave discharges are achieved by directly coupling microwave energy through a microwave-passing window into a discharge chamber containing a gas.
The existing technologies, e.g., either inductively coupled plasma or microwave plasmas, can have a limited operating range. To achieve high abatement efficiency, gases to be abated are typically excited and reacted in the plasma. It is desirable for the plasma to cover as much of the gas flow path as possible such that gas molecules cannot bypass the plasma region without interacting with the plasma.
During abatement, a pressure increase can be generated, for example, due to restrictions to the gas flow path. Since an abatement device is often located downstream of a process chamber in which the gases to be abated are used and/or generated, a pressure rise in the abatement device can directly impact the processes in the process chamber. Therefore, it is desirable to limit pressure rises during abatement.
Existing plasma abatement devices can suffer from limited ranges of gas flow rate and operating pressure. They can suffer from surface erosions due to, for example, reactive plasma chemistries and high energy ions generated in the plasma sources.